When developing and testing electrical or electronic components, assemblies, and equipment, it must be possible to sense signals from any points of a test piece.
In the case of electric voltages, for example, oscilloscopes are often used for detection and graphic display. Since almost all oscilloscopes are tabletop devices, they cannot be brought directly into contact with the test object; probes are often used for this purpose.
A probe in that regard is a measuring adapter for connection of a measuring device to a measuring point in a circuit. The task of the probe is the most undistorted possible transmission of the measured value, to the extent possible without noticeably influencing the test piece. An oscilloscope probe comprises the following parts: A probe tip to be brought into contact with the measuring signal on a conducting tract, contacts of a component, and the like; a short, flexible wire with a terminal to sense the reference potential of the measuring signal; a sensor body to be held with the hand; and cables and plug connections for transmission of the measuring signal to the oscilloscope.
There are requirements that are imposed on all probes: A low influence on the measuring signal, particularly a low input capacity, forwarding of the measuring signal to the oscilloscope in a way that is true to the original, i.e., low distortions of the signal form, and low amplitude errors and a high dynamic range should be mentioned here.
Measurements of electric voltages must be taken and transmitted to the oscilloscope in bipolar fashion. One pole can be called “measuring signal” and the other can be called “reference potential of the measuring signal.”
The reference potential of the measuring signal is looped through from the test object to the oscilloscope (oscilloscope reference potential). This pole of the oscilloscope is usually connected to the housing and also to the protective conductor of the power supply grid. In the case of multi-channel measurements, the reference potentials of the measuring signals are connected to each other via the oscilloscope.
If the test object has multiple reference potentials for the measuring signals and if they may not be connected to each other or to the protective conductor of the power supply grid, such measurements would have to be dispensed with.
Therefore, the galvanically decoupled transmission of measuring signals is needed for many technical measurement tasks. Examples are oscilloscope measurements on power supplies on the primary and secondary side or in the control and power circuit of electric drives.
Galvanic separation is understood to mean the case in which there is no way for charge carriers to flow from one electric circuit to another, immediately adjacent electric circuit. The most frequent application for galvanic separation is transformers with a connection to the public power grid. Galvanic separation that is produced by two electrically separated coils having a common iron core is required in that case.
An exchange of information between galvanically separated electric circuits is possible using non-electrical transmitters, for example opto-couplers (optical) or transformers (inductive).
In the case of probes for oscilloscopes, differential probes and optoelectronic probe systems provide “virtual” or genuine decoupling of the measuring signal and the oscilloscope. When the decoupling of the measuring signal from the oscilloscope is not necessary, probes will be used in which the measuring signal reference potential is looped through to the oscilloscope. Such probes are above all more economical than the usual differential probes or optoelectronic probe systems that are available on the market.
Differential probes function in accordance with the following principle: The output signal of two probe styluses is supplied on a differential amplifier located in the probe. Or two probes are attached to a differential amplifier over lines. The differential amplifier output is connected to the oscilloscope.
In the ideal case, both differential signals are completely decoupled from the reference potential of the oscilloscope by the differential amplifier. However, these systems have the disadvantage of a finite common-mode input voltage range. The finite common-mode input voltage range of the differential amplifier limits the amplitude of the common-mode signal. Moreover, the common-mode suppression falls as the frequency of the common-mode signal increases. Whereas, in common-mode signals of low frequency (e.g. 50 Hz), good common-mode suppression is achieved even using low-cost differential probes, they are often unusable for common-mode signals of high frequency or voltage increase speed.
Radiation is also disadvantageous in the case of differential probes. Both probes are generally connected to the differential amplifier by cable. As a result, portions of the measuring signal and its reference potential (common-mode signal) are emitted into the environment. As the frequency and amplitude of the common-mode signal rise, the radiation increases. This radiation loads the measuring signal and its reference signal, it is changed unintentionally, and the measured value, even with perfect transmission of the signal, no longer represents the signal to be measured in its undisturbed state.
By employing high-quality components and complex circuitry, some measurements in which the reference potential of the measuring signal is not connected to the reference signal of the oscilloscope can be improved using “high-end” differential probes. However, it is disadvantageous that these solutions are very expensive.
Systems that use optical transmission of the measuring signal are also known. In them, the measuring signal is modulated on an optical substrate, for example by modulation of the radiant power of a semiconductor laser diode. The modulated light is transmitted over an optical fiber. It is demodulated again in the receiver, for example by means of photodiodes. This method allows practically complete decoupling of the measuring signal from the oscilloscope. The optical signal is not prone to interference and does not cause interference.
DE 101 01 632 B4 and WO 89/09413 A1, as well as U.S. Pat. No. 5,465,043, contain various laser-based methods with the use of optical waveguides.
DE 101 01 632 B4 presents an oscilloscope probe head with a fiber optic sensor for galvanically separated detection of electric variables.
WO 89/09413 A1 describes an oscilloscope probe head with a fiber optic sensor for galvanically separated detection of electric variables. The sensor head has an electro-optic crystal by means of which an externally-applied light is influenced by the effect of an electrical field using polarization. The difference between the original polarization and the changed polarization is then converted into an electric variable. Optical fibers are used for transmission.
A probe for potential- and interference-free detection of the intensity of an electric field or the absolute value of a voltage is disclosed in U.S. Pat. No. 5,465,043.
Here, too, the change in the light polarization reflects the field strength or the voltage value.
This involves a relatively complex method with regard to the use of components and manufacturing.
For example, WO 89/09413 A1 requires in the probe an optical system that has elements that must be mechanically fastened and adjusted with regard to their axis, such as a beamsplitter, a mirror, and lenses, and an expensive electro-optical sensor is used in DE 101 01 632 B4.
Other solutions of prior art for galvanically decoupled transmission over optical fibers require on the sensor side an electrical voltage supply for the electronic components such as laser, LED, or amplifier. Providing them in a way that is galvanically decoupled represents an additional difficulty, for example due to the requirement for more space and the limited functional life of a battery. Inductive transmission types are limited as to their dielectric strength and generate a parasite capacitative coupling between the sensor and the evaluation device.